1. Field of the Invention
The present invention relates to piezoelectric (PZT) ceramic compositions. More particularly, it relates to low-loss PZT ceramic compositions cofirable with silver at a reduced sintering temperature and to methods for producing such compositions.
2. Description of Related Art
The low-temperature sintering of lead-based piezoelectric ceramics has been studied extensively by those in the electronic materials industry. For example, it has been found that through the addition of various frits, glass additives or softening agents, the sintering temperature of lead zirconate titanate (PZT) ceramics may be reduced from about 1250.degree. C. to about 960.degree. C. Furthermore, it has been found that, in general, the sintering temperature of a lead based piezoelectric ceramic may be reduced by doping the piezoelectric composition with low-melting oxides.
Reference is made to an article titled "Low-Temperature Sintering of Lead-Based Piezoelectric Ceramics" by Gui Zhilun et al., Department of Chemical Engineering, Tsinghua University, Beijing, China (J. Am. Ceram. Soc., 72[3] 486-91 (1989)). This article discusses how to reduce the sintering temperature of a PZT composition by the addition of a small amount of a lower melting frit, B.sub.2 O.sub.3 --Bi.sub.2 O.sub.3 --CdO, while maintaining desirable electrical properties. Although this article discusses hard (low-loss) PZT ceramic materials, the sintering temperature is still too high for large scale manufacturing of PZT ceramics cofirable with a pure silver electrode material. As such, this composition may be of limited use in a large scale production environment.
U.S. Pat. No. 5,433,917 issued Jul. 18, 1995 to Srivastava et al. teaches the manufacture of a morphotropic PZT ceramic composition having reduced sintering temperatures and a process for producing the same. In this patent, the PZT is sintered with an effective amount of an eutectic mixture of copper oxide (CuO) and an oxide of an alkaline earth metal, preferably barium oxide and/or strontium oxide, to reduce the sintering temperature of the PZT composition to about 1000.degree. C.
Although this patent discusses the addition of CuO as part of the frit, it deals primarily with soft-PZT materials which typically have high loss characteristics and are typically not intended for use in high power applications. This patent also addresses only a very narrow field of compositions, namely morphotropic lead zirconium titanate piezoelectric ceramic compositions, containing about 52(mole or atomic) % lead zirconate and 48(mole or atomic) % lead titanate.
It has been found that the compositions taught in the U.S. Pat. No. 5,433,917 patent are of limited use in the large scale manufacture of piezoelectric ceramic products such as piezoelectric multilayer transformers where low-loss properties accompanied by moderate coupling capabilities are desirable. In particular, a sintering temperature in the range of about 950.degree. C. still poses major cofiring problems with conventional silver (Ag) compositions, which typically have a melting temperature of about 962.degree. C., and which are used to form the electrode patterns in standard multilayer packages. Potential problems during the sintering process may include, but are not limited to, silver-ceramic reactions at boundary layers, volatile silver vapor resulting in bubbles or trapped gases, uneven or incomplete shrinkage or densification, degradation of electrical properties, or delamination, all leading to possible product failure.
To combat these adverse effects of sintering so close to the melting point of Ag, many manufacturers have chosen to employ a silver-palladium (Ag--Pd) electrode composition. Ag--Pd compositions have a melting point which is dependent upon the Pd content in the composition. For example, a 90% Ag-10% Pd composition has a melting temperature of about 1020.degree. C. This higher temperature processing can become very expensive compared to the processing costs associated with pure Ag electrode compositions. Higher sintering temperatures necessitate the use of more expensive noble metals such as Pt, Pd, Au, or their alloys (which have higher melting points) as the internal electrode material. Thus, use of lower sintering temperature PZT materials may result in substantial savings in the cost of electrode materials as well as substantial savings in energy necessary for high temperature firing.
Commonly assigned, U.S. Pat. No. 5,792,379, issued to the same inventors as the present application, discloses a low loss PZT composition capable of being cofired with a silver electrode layer at temperatures of about 900.degree. C. The composition consists of a main composition and an additive, the main composition being PZT and the additive comprising a glass frit composed of a combination of B.sub.2 O.sub.3, Bi.sub.2 O.sub.3, and some other metal oxide (MeO). However, the composition disclosed in the U.S. Pat. No. 5,792,379 patent employs a glass frit additive that adds extra steps which involve the making of glass and cost to the production of the PZT composition. Thus, it would be desirable to provide a PZT composition that can be formulated without the use of a glass additive.
Another problem encountered with prior art compositions is that the low-melting additions, which decrease the sintering temperature, also may modify the properties of the ceramics, causing degradation of the electrical properties of the composition which render these compositions impractical for their intended purposes as low-loss devices.